Conventional color diffusion transfer photographic film units can be broadly classified into peel-apart type units and units which are used without being peeled-apart. The peel apart type units have a photo-sensitive layer and a dye image receiving layer established by coating each layer on separate supports and, after exposure in the form of an image, the photosensitive element and the dye image receiving element are laminated together with a processing composition spread between them, and subsequently the dye image which has been transferred to the dye image receiving layer is obtained by peeling-away the dye image receiving element.
The distinguishing feature of such an embodiment is that there is no loss of image quality of the sort seen with the type of unit which is not peeled-apart as described hereinafter, and extremely good color reproduction can be achieved because the dye image which has been formed on the image receiving layer which has been established by coating on a support is viewed directly. On the other hand, such a unit is inconvenient when operating a camera in that the photosensitive element and the image receiving element have to be laminated together in the camera. Furthermore, the elements are sticky from the alkaline processing fluid after peeling apart and they are liable to stick to the surroundings, and this is inconvenient when handling processed film.
Color diffusion transfer photographic film units of which the distinguishing features are that they are comprised of a photosensitive element which has established sequentially on a white support at least (a) a layer which has a neutralizing function, (b) a dye image receiving layer, (c) a peeling layer and (d) at least one silver halide emulsion layer which is combined with a dye image forming substance; an alkali processing composition which contains a light shielding agent; and a transparent cover sheet have been disclosed in JP-A-63-226649, and color diffusion transfer photographic film units comprised of a photosensitive element which has established sequentially on a white support at least (a) a dye image receiving layer (b) a peeling layer and (c) at least one silver halide emulsion layer which is combined with a dye image forming substance; an alkali processing composition which contains a light shielding agent; and a transparent cover sheet which has a layer which has at least a neutralizing function on the side on which the processing composition is spread have been disclosed is Japanese Patent Application No. 63-120201 (corresponding to JP-A-1-289950), and in each case a new peel-apart type photographic film unit has been suggested with a view to resolving the aforementioned problems. (The term "JP-A" as used herein signifies an "unexamined published Japanese patent application".) Moreover, peel-apart type color diffusion transfer photographic film units which have a dye image receiving layer and at least one silver halide emulsion layer which is combined with a dye image forming substance on the same support and which have a peeling layer between the dye image receiving layer and the silver halide emulsion layer have also disclosed in U.S. Pat. Nos. 4,529,683, 4,499,174, 4,401,746, 3,730,718 and 3,227,550.
When a non-diffusible dye releasing redox compound is used for the dye image forming substance and pyrazolidones are used as the developing agent in the preferred embodiments of these peel-apart type photographic film units, staining can occur after peeling apart. The peel surface pH immediately after peeling apart is preferably not more than 10, and most desirably not more than 8 but, of course, when there is no neutralizing layer, and even when a neutralizing layer is provided in the film unit, there are cases in which the peel surface pH immediately after peeling apart is not lowered enough. That is to say, there are, for example, cases in which the pH is not lowered adequately because the peeling apart has been carried out a little before the preferred peeling time, and cases in which the pH falls gradually after peeling apart but in which the pH has not fallen completely immediately after peeling apart. The developing agent on a peeled sheet surface which includes the dye image receiving layer which has been peeled apart in a condition such as this where the pH has not fallen is subjected to aerial oxidation and in some cases pink or yellow-brown staining is seen to occur, and this results in a marked loss of picture quality.